maintain an up-to-date mirror of forge.softwareheritage.org code on GitHub

We have a under-utilized GitHub account.

The main development of Software Heritage will remain on forge.softwareheritage.org for the foreseeable future, but we should nonetheless automatically maintain on GitHub a complete up-to-date mirror of our code, so that GitHub users can easily submit pull requests. While dealing with pull requests when GitHub is not our main platform is somewhat painful, we really need to lower the bar for code contributions, and having a presence on GitHub might help in that respect.

@olasd: can you please briefly document here how the few repos we currently have on GitHub are currently automatically sync'd? That will make this task more actionable. TIA.

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Migrated from T619 (view on Phabricator)